The present invention relates to a method for operating a fundamental frequency clocked converter on the power supply side comprising a bridge circuit equipped with controllable semiconductor switches, and to an apparatus provided for implementing the method, that is to say a converter device comprising a converter of this type and a bridge circuit of this type.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
A converter device or a converter circuit has at least one power supply connection and phase sections which are provided therefor as power supply connection and function as current paths, in other words three phase sections in the case of connection to a three-phase power supply, then the actual converter with the bridge circuit that implements the conversion functionality, and an DC link circuit at the output of the converter, wherein a first output of the converter forms a first DC link circuit contact and a second output of the converter forms a second DC link circuit contact. An DC link voltage is present between these two DC link circuit contacts during operation, said DC link voltage arising as a DC voltage on account of the conversion. For the purpose of buffering and smoothing the DC link circuit voltage, an DC link circuit capacitance is usually arranged between the DC link circuit contacts.
In the case of a positive power flow arising during operation into the intermediate circuit, the converter operates approximately like a so-called B6 diode bridge, since only the diodes of the converter carry the current, while transistors that are respectively reverse-connected in parallel therewith as semiconductor switches are deenergized. Conversely, in the case of power flow from the DC link circuit into the power supply, the diodes are reverse-biased and precisely one path into the power supply is made possible for the current by corresponding activation of individual transistors at each point in time. One preferred circuit for the bridge circuit forming the converter is a so-called IGBT (insulated gate bipolar transistor) B6 bridge, which has a passive parallel diode bridge that enables purely passive operation in one power direction, that is to say e.g. for the power flow into the intermediate circuit, and an active feed-in/feedback power flow, that is to say correspondingly e.g. from the DC link circuit into the power supply. In fundamental frequency operation, switching edges for driving the transistors, which are referred to hereinafter generally as semiconductor switches, are at so-called natural triggering instants, that is to say intersection points of the power supply voltages which are fed to the circuit or to the device via the individual current paths, that is to say, in the case of a three-phase power supply, the individual lines—designated hereinafter by R, S, T—of the three-phase power supply with the power supply voltages UR, US and UT fed by means thereof.
Fundamental frequency clocked operation of converters is distinguished by some advantages that make it expedient to use. Since the semiconductor switches, which can also be referred to as current valves, are not clocked at high frequency, this results for example in low switching losses and furthermore in the possibility of using simple and cost-effective commutation inductors and power supply filters, and finally in low excitation of system oscillations that can lead e.g. to high motor bearing currents. Finally, the operation of converters of this type is generally robust with respect to power supply disturbances, power supply unbalances and the like.
On account of the properties mentioned, fundamental frequency clocked converters are suitable in particular in areas of application in which cost-effective solutions are sought and high cost pressure prevails. Therefore, there is a particular demand for cost reducing measures. In this regard, changes to construction or circuit are particularly effective since they take effect in production for each individual device and thereby enable large savings in total.
As already described, the switching edges of the semiconductor switches during fundamental frequency operation are at the natural triggering instants defined by the intersection points of the power supply voltages. Therefore, knowledge of the power supply phase angle is necessary for the correct driving of the semiconductor switches. For this purpose, measurement methods proposed to date are comparatively complicated and include, e.g., an evaluation of a voltage between lines in the converter or a detection of the instant of a change in sign of these voltages with the aid of a comparator circuit. The measuring devices required for such measurements are not necessary either for the actual converter function or for the protection of the converter and could be omitted in this respect if it were possible to implement a different solution for determining the power supply phase angle.
It is an object of the present invention, therefore, to specify a novel, in which method in particular the circuitry outlay for a metrological detection is intended to be minimized, in particular is intended to be obviated, while measurement variables, specifically measurement variables which relate to the phase currents and which are also required for device protection, are intended to be used.
It would therefore be desirable and advantageous to obviate prior art shortcomings by providing an improved method for determining the power supply phase angle for instances of fundamental frequency clocked feed-in/feedback and to thereby reduce the measurement complexity.